Flotation is a physical-chemical process for the separation of materials which uses the water repellent or water affinity surface characteristics of the materials to be separated. It is the most commonly used method for the concentration of basic sulfides of metals such as copper, molybdenum, lead, zinc. etc. that have a natural hydrophobicity, which increased with the application of flotation reagents. It consists basically in placing the materials to be separated previously ground and treated with flotation reagents in an aqueous medium to form a pulp into which a gas has been introduced to generate bubbles. Hydrophobic materials adhere to the surface of the bubbles and hydrophilic materials remain within the pulp. The froth carrying the hydrophobic materials rises to the surface and overflows into the froth collecting troughs wherefrom it is removed from the flotation apparatus.
There is currently a large variety of flotation apparatus in various shapes, sizes and methods for the injection of gas into the pulp. Whatever their design however, they must perform the following basic functions:                introduce to the machine the gas and aqueous pulp containing the materials to be separated;        maintain the pulp and materials to be separated in suspension by avoiding sedimentation and the accumulation of solids in the apparatus;        generate gas bubbles and provoke their contact with the hydrophobic materials to be concentrated;        provide a system for the withdrawal of the froth loaded with the hydrophobic substances constituting the concentrate; and        maintain the materials with water affinity inside the pulp which constitutes the tailings and provide for a discharge system.        
The machines performing these functions can be classified in two main group depending on how they perform these functions:                machines with motorized mechanic agitation, and        pneumatic machines without mechanical agitation.        
Machines with motorized mechanical agitation are characterized in that they create contact of the bubbles with the solid hydrophobic and maintain the pulp in suspension by means of a rotor-stator assembly. The pulp is fed through the lower part of a tank in whose central axle the rotor-stator is installed. The rotor is activated by a motorized system installed at the upper end of its axle and is also used to introduce the gas. The tank has in its upper section ridges for froth overflows which are discharged by froth collecting troughs. The extraction of the tailings goes through the lower part opposite the feeding end with valves which control the flow and level of the pulp within the tank. Depending on whether the air is introduced to the flotation machine with an external system or is introduced by the rotor itself these machines are classified as forced aeration or auto aspiration machines. A number of flotation machines are used in mineral concentration operating in series and with residence times varying between 20 and 40 minutes.
Mechanical agitation machines are the leaders in the world mining industry markets and differ mainly only in the design of the rotor-stator system. Pneumatic flotation machines do not have a motorized mechanical agitation system and flotation columns are the ones in most widespread use in the mining industry. The columns have two clearly defined areas: one for collection and one for a cleaning area. The feeding of the pulp is effected under the interface of these two areas. The bubbles are generated by injecting gas through gas nozzles placed at the bottom of the column. As the hydrophobic particles descend they become exposed in the collection area to ascending bubbles, which they join and are then transported for cleaning to the upper area of the column where a thick layer of froth is formed to which wash water is added to obtain clean concentrates. The hydrophilic particles do not adhere to the bubbles and continue descending with the aqueous pulp and are finally discharged from the bottom through a flow control valve. Flotation columns have in common their tallness, normally more than 10 meters. For this reason the air injected at the bottom under the pressure of 30 psi. needed to overcome the pressure in the pulp column and the load loss in the bubbler. Additionally, because of their great height they also normally require pumps to supply the pulp. The differences between the columns is found in the design of the bubblers which inject the gas. Depending on the location of the gas bubblers they can be classified as internal bubbler columns and external bubbler generators. In columns with internal nozzles the bubble generator is in the lower part of the column and in direct contact with the pulp. The nozzles can be metallic, of porous ceramic, filter fabrics and perforated rubber. The drawback in these types of bubble generators is that the openings wherefrom the gas is issued gets clogged with pulp particles and/or encrustations due to the hardness of the water used in the mining industry. Replacing them requires withdrawing them from the column, something which causes a drop in the availability of these equipments. In columns with external bubble generators these are located outside the column. Part of the pulp tailing is recycled with pumps, combining static gas-pulp mixers discharge of which is injected into the lower portion of the column. The static generators generate a high load loss and accelerated wear. Another type of external bubble generator instead of recycling the tailings pulp injects water which is mixed with the gas in a static mixer. An example of this type of bubble generator is the one developed in the U.S. Pat. No. 4,752,383, where the static generator is a container holding small glass beads so packed as to impede their displacement. Water and air are mixed in their transit through the packed layer of glass beads, and the gas-water mixture is discharged at the other end of the apparatus and injected through the lower area of the column The layer of glass beads packed in this type of bubblers becomes encrusted with increasing rapidity by the hardness of the water used by the mining industry and ends up clogging the bubbler. The required pressure for water and air injection to this type of bubblers is normally above 50 psi, which must be increased as the bubbler becomes clogged by encrustations, a reason for which they are no longer used.